Gas chromatography is the separation of a mixture of chemical compounds due to their migration rates through a chromatographic column. This separates the compounds based on differences in boiling point, polarity, or molecular size. The separated compounds then flow across a suitable detector, such as a thermal conductivity detector (TCD) that determines the concentration of each compound represented in the overall sample. Knowing the concentration of the individual compounds makes it possible to calculate certain physical properties such as BTU or specific gravity using industry-standard equations.
A gas chromatograph is an analyzer that passes a small volume of gas through chromatographic columns to separate and individually measure the unique gas components of the sample mixture. The analysis cycle can be split into two general phases. The first phase is a sample injection phase, and the second phase is the separation and measurement phase. Each analysis cycle requires a certain amount of time to execute depending on the application, typically measured in minutes.
An online gas chromatograph will perform sequential analysis runs of a single or multiple sample streams. After the sample injection phase of the analysis cycle, the next stream gas to be analyzed is then purged through the internal sample flow paths and the sample loop until the end of the analysis cycle to ensure that there is fresh, uncontaminated sample in the sample loop for injection into the columns at the beginning of the next analysis cycle.
A calibration sequence consists of three or more consecutive analysis cycles of calibration gas with a known composition that is used to calculate the individual calibration factors (known as “response factors”) for each component measured by the gas chromatograph. In the natural gas transmission market segment where gas chromatographs are used as part of the custody transfer metering system, the gas chromatograph is typically set to automatically calibrate once a day to ensure the accuracy of the measurement, and to validate the correct operation of the analyzer. The calibration gas is typically a custom blended gas that has a certified composition. The cost of the calibration gas can be a significant component of the operating costs of the gas chromatograph. Conserving the amount of calibration gas consumed by gas chromatograph operation would reduce the overall costs of such online gas chromatography.